I was a previous beta tester just I think my keys were old.Keep in mind that a) not all beta tester applications are accepted and you won't get a response if it was rejected and b) if you are accepted it could take awhile to get a response. Of course this is me assuming they're actually still doing the beta tester program where they issue beta tester licenses to specific applicants, which they may not be anymore. I've heard from a few users that they haven't issued beta tester licenses in a good while, so there's a chance that the beta tester license program may have indeed ended (and I wouldn't be surprised if they did end it, it's actually not needed anymore and it was prone for abuse from those looking for a free license with no interest in beta testing).
This doesn't mean that beta testing AdGuard apps is over, they changed it a few years ago where anyone can beta test most AG apps (except iOS) by setting AG's update channel from stable to beta or even nightly or with AG for Mac you have to manually change from stable to beta or nightly. There's no special beta tester licenses or anything else needed either... well, other than a legit AG license to use it beyond the 14 day trial.